Data transmission rate is rapidly growing these days. The high data transmission rate is mainly due to advent and wide deployment of machine-to-machine (M2M) communication and various types of devices requiring high data transmission rate such as smart phones, tablet PCs, and the like. To meet the high data transmission rate required, novel technologies are recently getting attention, including carrier aggregation technology, cognitive radio technology, and the like which utilize more frequency bands efficiently; and multiple antenna technology, multiple base station collaboration technology, and the like for increasing data capacity with limited frequency resources.
Also, the current trend for wireless communication networks is toward increasing density of nodes that can be accessed in the vicinity of the user′ current location. Here, a node stands for an antenna or a group of antennas spaced apart by more than a predetermined distance from each other in a distributed antenna system (DAS); however, it can be used in a broader sense rather than limited to the above definition. In other words, a node may correspond to a pico-cell base station (PeNB), a home base station (HeNB), a remote radio head (RRH), a remote radio unit (RRU), a repeater, and so on. A wireless communication system incorporating high density of nodes can provide a lot higher system performance due to collaboration among the nodes. That is, compared with the case where each node operates as an independent base station (namely, a base station (BS), an advanced BS (ABS), a Node-B (NB), an eNode-B (eNB), an access point (AP), and so on) showing no collaboration between each other, if one control station manages transmission and reception of individual nodes and thus, the nodes operate as an antenna or a group of antennas in a cell, vastly superior performance can be achieved. In what follows, a wireless communication system incorporating a plurality of nodes is called a multiple node system.
A node can be defined not only for a group of antennas spaced apart at regular intervals but also for an arbitrary group of antennas regardless of intervals among the antennas. For example, a base station comprising cross polarized antennas can be regarded to consist of nodes of H-pol antennas and nodes of V-pol antennas.
In a multiple node system, different nodes may be used to transmit signals according to the respective user equipment types; at the same time, a plurality of nodes may be employed for the transmission. At this time, reference signals different from each other can be transmitted for each of the node types. In this case, a user equipment measures channel status between nodes by using a plurality of reference signals and channel status information can be obtained as feedback periodically or aperiodically.